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/* |
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* Written by Doug Lea with assistance from members of JCP JSR-166 |
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* Expert Group and released to the public domain, as explained at |
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* http://creativecommons.org/publicdomain/zero/1.0/ |
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*/ |
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|
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package java.util.concurrent; |
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|
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import java.util.concurrent.locks.Condition; |
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import java.util.concurrent.locks.ReentrantLock; |
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|
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/** |
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* A synchronization aid that allows a set of threads to all wait for |
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* each other to reach a common barrier point. CyclicBarriers are |
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* useful in programs involving a fixed sized party of threads that |
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* must occasionally wait for each other. The barrier is called |
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* <em>cyclic</em> because it can be re-used after the waiting threads |
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* are released. |
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* |
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* <p>A {@code CyclicBarrier} supports an optional {@link Runnable} command |
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* that is run once per barrier point, after the last thread in the party |
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* arrives, but before any threads are released. |
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* This <em>barrier action</em> is useful |
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* for updating shared-state before any of the parties continue. |
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* |
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* <p><b>Sample usage:</b> Here is an example of using a barrier in a |
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* parallel decomposition design: |
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* |
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* <pre> {@code |
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* class Solver { |
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* final int N; |
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* final float[][] data; |
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* final CyclicBarrier barrier; |
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* |
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* class Worker implements Runnable { |
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* int myRow; |
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* Worker(int row) { myRow = row; } |
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* public void run() { |
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* while (!done()) { |
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* processRow(myRow); |
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* |
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* try { |
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* barrier.await(); |
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* } catch (InterruptedException ex) { |
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* return; |
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* } catch (BrokenBarrierException ex) { |
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* return; |
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* } |
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* } |
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* } |
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* } |
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* |
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* public Solver(float[][] matrix) { |
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* data = matrix; |
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* N = matrix.length; |
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* Runnable barrierAction = () -> mergeRows(...); |
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* barrier = new CyclicBarrier(N, barrierAction); |
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* |
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* List<Thread> threads = new ArrayList<>(N); |
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* for (int i = 0; i < N; i++) { |
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* Thread thread = new Thread(new Worker(i)); |
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* threads.add(thread); |
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* thread.start(); |
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* } |
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* |
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* // wait until done |
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* for (Thread thread : threads) |
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* thread.join(); |
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* } |
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* }}</pre> |
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* |
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* Here, each worker thread processes a row of the matrix then waits at the |
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* barrier until all rows have been processed. When all rows are processed |
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* the supplied {@link Runnable} barrier action is executed and merges the |
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* rows. If the merger |
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* determines that a solution has been found then {@code done()} will return |
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* {@code true} and each worker will terminate. |
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* |
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* <p>If the barrier action does not rely on the parties being suspended when |
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* it is executed, then any of the threads in the party could execute that |
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* action when it is released. To facilitate this, each invocation of |
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* {@link #await} returns the arrival index of that thread at the barrier. |
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* You can then choose which thread should execute the barrier action, for |
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* example: |
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* <pre> {@code |
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* if (barrier.await() == 0) { |
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* // log the completion of this iteration |
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* }}</pre> |
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* |
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* <p>The {@code CyclicBarrier} uses an all-or-none breakage model |
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* for failed synchronization attempts: If a thread leaves a barrier |
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* point prematurely because of interruption, failure, or timeout, all |
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* other threads waiting at that barrier point will also leave |
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* abnormally via {@link BrokenBarrierException} (or |
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* {@link InterruptedException} if they too were interrupted at about |
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* the same time). |
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* |
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* <p>Memory consistency effects: Actions in a thread prior to calling |
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* {@code await()} |
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* <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a> |
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* actions that are part of the barrier action, which in turn |
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* <i>happen-before</i> actions following a successful return from the |
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* corresponding {@code await()} in other threads. |
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* |
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* @see CountDownLatch |
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* |
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* @author Doug Lea |
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* @since 1.5 |
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*/ |
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public class CyclicBarrier { |
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/** |
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* Each use of the barrier is represented as a generation instance. |
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* The generation changes whenever the barrier is tripped, or |
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* is reset. There can be many generations associated with threads |
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* using the barrier - due to the non-deterministic way the lock |
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* may be allocated to waiting threads - but only one of these |
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* can be active at a time (the one to which {@code count} applies) |
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* and all the rest are either broken or tripped. |
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* There need not be an active generation if there has been a break |
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* but no subsequent reset. |
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*/ |
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private static class Generation { |
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boolean broken; // initially false |
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} |
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|
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/** The lock for guarding barrier entry */ |
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private final ReentrantLock lock = new ReentrantLock(); |
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/** Condition to wait on until tripped */ |
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private final Condition trip = lock.newCondition(); |
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/** The number of parties */ |
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private final int parties; |
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/** The command to run when tripped */ |
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private final Runnable barrierCommand; |
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/** The current generation */ |
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private Generation generation = new Generation(); |
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|
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/** |
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* Number of parties still waiting. Counts down from parties to 0 |
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* on each generation. It is reset to parties on each new |
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* generation or when broken. |
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*/ |
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private int count; |
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|
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/** |
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* Updates state on barrier trip and wakes up everyone. |
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* Called only while holding lock. |
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*/ |
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private void nextGeneration() { |
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// signal completion of last generation |
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trip.signalAll(); |
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// set up next generation |
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count = parties; |
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generation = new Generation(); |
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} |
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|
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/** |
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* Sets current barrier generation as broken and wakes up everyone. |
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* Called only while holding lock. |
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*/ |
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private void breakBarrier() { |
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generation.broken = true; |
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count = parties; |
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trip.signalAll(); |
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} |
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|
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/** |
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* Main barrier code, covering the various policies. |
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*/ |
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private int dowait(boolean timed, long nanos) |
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throws InterruptedException, BrokenBarrierException, |
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TimeoutException { |
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final ReentrantLock lock = this.lock; |
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lock.lock(); |
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try { |
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final Generation g = generation; |
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|
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if (g.broken) |
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throw new BrokenBarrierException(); |
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|
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if (Thread.interrupted()) { |
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breakBarrier(); |
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throw new InterruptedException(); |
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} |
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|
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int index = --count; |
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if (index == 0) { // tripped |
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boolean ranAction = false; |
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try { |
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final Runnable command = barrierCommand; |
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if (command != null) |
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command.run(); |
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ranAction = true; |
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nextGeneration(); |
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return 0; |
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} finally { |
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if (!ranAction) |
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breakBarrier(); |
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} |
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} |
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|
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// loop until tripped, broken, interrupted, or timed out |
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for (;;) { |
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try { |
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if (!timed) |
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trip.await(); |
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else if (nanos > 0L) |
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nanos = trip.awaitNanos(nanos); |
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} catch (InterruptedException ie) { |
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if (g == generation && ! g.broken) { |
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breakBarrier(); |
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throw ie; |
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} else { |
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// We're about to finish waiting even if we had not |
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// been interrupted, so this interrupt is deemed to |
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// "belong" to subsequent execution. |
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Thread.currentThread().interrupt(); |
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} |
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} |
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|
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if (g.broken) |
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throw new BrokenBarrierException(); |
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|
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if (g != generation) |
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return index; |
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|
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if (timed && nanos <= 0L) { |
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breakBarrier(); |
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throw new TimeoutException(); |
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} |
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} |
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} finally { |
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lock.unlock(); |
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} |
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} |
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|
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/** |
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* Creates a new {@code CyclicBarrier} that will trip when the |
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* given number of parties (threads) are waiting upon it, and which |
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* will execute the given barrier action when the barrier is tripped, |
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* performed by the last thread entering the barrier. |
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* |
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* @param parties the number of threads that must invoke {@link #await} |
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* before the barrier is tripped |
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* @param barrierAction the command to execute when the barrier is |
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* tripped, or {@code null} if there is no action |
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* @throws IllegalArgumentException if {@code parties} is less than 1 |
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*/ |
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public CyclicBarrier(int parties, Runnable barrierAction) { |
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if (parties <= 0) throw new IllegalArgumentException(); |
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this.parties = parties; |
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this.count = parties; |
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this.barrierCommand = barrierAction; |
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} |
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|
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/** |
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* Creates a new {@code CyclicBarrier} that will trip when the |
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* given number of parties (threads) are waiting upon it, and |
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* does not perform a predefined action when the barrier is tripped. |
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* |
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* @param parties the number of threads that must invoke {@link #await} |
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* before the barrier is tripped |
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* @throws IllegalArgumentException if {@code parties} is less than 1 |
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*/ |
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public CyclicBarrier(int parties) { |
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this(parties, null); |
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} |
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|
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/** |
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* Returns the number of parties required to trip this barrier. |
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* |
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* @return the number of parties required to trip this barrier |
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*/ |
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public int getParties() { |
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return parties; |
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} |
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|
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/** |
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* Waits until all {@linkplain #getParties parties} have invoked |
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* {@code await} on this barrier. |
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* |
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* <p>If the current thread is not the last to arrive then it is |
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* disabled for thread scheduling purposes and lies dormant until |
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* one of the following things happens: |
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* <ul> |
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* <li>The last thread arrives; or |
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* <li>Some other thread {@linkplain Thread#interrupt interrupts} |
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* the current thread; or |
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* <li>Some other thread {@linkplain Thread#interrupt interrupts} |
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* one of the other waiting threads; or |
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* <li>Some other thread times out while waiting for barrier; or |
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* <li>Some other thread invokes {@link #reset} on this barrier. |
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* </ul> |
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* |
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* <p>If the current thread: |
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* <ul> |
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* <li>has its interrupted status set on entry to this method; or |
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* <li>is {@linkplain Thread#interrupt interrupted} while waiting |
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* </ul> |
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* then {@link InterruptedException} is thrown and the current thread's |
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* interrupted status is cleared. |
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* |
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* <p>If the barrier is {@link #reset} while any thread is waiting, |
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* or if the barrier {@linkplain #isBroken is broken} when |
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* {@code await} is invoked, or while any thread is waiting, then |
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* {@link BrokenBarrierException} is thrown. |
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* |
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* <p>If any thread is {@linkplain Thread#interrupt interrupted} while waiting, |
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* then all other waiting threads will throw |
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* {@link BrokenBarrierException} and the barrier is placed in the broken |
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* state. |
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* |
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* <p>If the current thread is the last thread to arrive, and a |
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* non-null barrier action was supplied in the constructor, then the |
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* current thread runs the action before allowing the other threads to |
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* continue. |
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* If an exception occurs during the barrier action then that exception |
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* will be propagated in the current thread and the barrier is placed in |
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* the broken state. |
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* |
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* @return the arrival index of the current thread, where index |
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* {@code getParties() - 1} indicates the first |
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* to arrive and zero indicates the last to arrive |
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* @throws InterruptedException if the current thread was interrupted |
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* while waiting |
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* @throws BrokenBarrierException if <em>another</em> thread was |
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* interrupted or timed out while the current thread was |
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* waiting, or the barrier was reset, or the barrier was |
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* broken when {@code await} was called, or the barrier |
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* action (if present) failed due to an exception |
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*/ |
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public int await() throws InterruptedException, BrokenBarrierException { |
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try { |
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return dowait(false, 0L); |
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} catch (TimeoutException toe) { |
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throw new Error(toe); // cannot happen |
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} |
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} |
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|
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/** |
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* Waits until all {@linkplain #getParties parties} have invoked |
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* {@code await} on this barrier, or the specified waiting time elapses. |
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* |
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* <p>If the current thread is not the last to arrive then it is |
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* disabled for thread scheduling purposes and lies dormant until |
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* one of the following things happens: |
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* <ul> |
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* <li>The last thread arrives; or |
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* <li>The specified timeout elapses; or |
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* <li>Some other thread {@linkplain Thread#interrupt interrupts} |
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* the current thread; or |
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* <li>Some other thread {@linkplain Thread#interrupt interrupts} |
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* one of the other waiting threads; or |
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* <li>Some other thread times out while waiting for barrier; or |
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* <li>Some other thread invokes {@link #reset} on this barrier. |
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* </ul> |
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* |
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* <p>If the current thread: |
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* <ul> |
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* <li>has its interrupted status set on entry to this method; or |
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* <li>is {@linkplain Thread#interrupt interrupted} while waiting |
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* </ul> |
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* then {@link InterruptedException} is thrown and the current thread's |
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* interrupted status is cleared. |
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* |
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* <p>If the specified waiting time elapses then {@link TimeoutException} |
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* is thrown. If the time is less than or equal to zero, the |
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* method will not wait at all. |
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* |
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* <p>If the barrier is {@link #reset} while any thread is waiting, |
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* or if the barrier {@linkplain #isBroken is broken} when |
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* {@code await} is invoked, or while any thread is waiting, then |
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* {@link BrokenBarrierException} is thrown. |
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* |
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* <p>If any thread is {@linkplain Thread#interrupt interrupted} while |
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* waiting, then all other waiting threads will throw {@link |
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* BrokenBarrierException} and the barrier is placed in the broken |
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* state. |
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* |
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* <p>If the current thread is the last thread to arrive, and a |
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* non-null barrier action was supplied in the constructor, then the |
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* current thread runs the action before allowing the other threads to |
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* continue. |
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* If an exception occurs during the barrier action then that exception |
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* will be propagated in the current thread and the barrier is placed in |
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* the broken state. |
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* |
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* @param timeout the time to wait for the barrier |
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* @param unit the time unit of the timeout parameter |
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* @return the arrival index of the current thread, where index |
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* {@code getParties() - 1} indicates the first |
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* to arrive and zero indicates the last to arrive |
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* @throws InterruptedException if the current thread was interrupted |
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* while waiting |
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* @throws TimeoutException if the specified timeout elapses. |
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* In this case the barrier will be broken. |
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* @throws BrokenBarrierException if <em>another</em> thread was |
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* interrupted or timed out while the current thread was |
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* waiting, or the barrier was reset, or the barrier was broken |
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* when {@code await} was called, or the barrier action (if |
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* present) failed due to an exception |
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*/ |
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public int await(long timeout, TimeUnit unit) |
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throws InterruptedException, |
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BrokenBarrierException, |
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TimeoutException { |
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return dowait(true, unit.toNanos(timeout)); |
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} |
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|
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/** |
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* Queries if this barrier is in a broken state. |
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* |
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* @return {@code true} if one or more parties broke out of this |
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* barrier due to interruption or timeout since |
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* construction or the last reset, or a barrier action |
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* failed due to an exception; {@code false} otherwise. |
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*/ |
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public boolean isBroken() { |
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final ReentrantLock lock = this.lock; |
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lock.lock(); |
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try { |
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return generation.broken; |
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} finally { |
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lock.unlock(); |
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} |
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} |
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|
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/** |
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* Resets the barrier to its initial state. If any parties are |
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* currently waiting at the barrier, they will return with a |
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* {@link BrokenBarrierException}. Note that resets <em>after</em> |
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* a breakage has occurred for other reasons can be complicated to |
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* carry out; threads need to re-synchronize in some other way, |
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* and choose one to perform the reset. It may be preferable to |
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* instead create a new barrier for subsequent use. |
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*/ |
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public void reset() { |
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final ReentrantLock lock = this.lock; |
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lock.lock(); |
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try { |
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breakBarrier(); // break the current generation |
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nextGeneration(); // start a new generation |
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} finally { |
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lock.unlock(); |
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} |
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} |
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|
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/** |
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* Returns the number of parties currently waiting at the barrier. |
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* This method is primarily useful for debugging and assertions. |
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* |
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* @return the number of parties currently blocked in {@link #await} |
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*/ |
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public int getNumberWaiting() { |
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final ReentrantLock lock = this.lock; |
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lock.lock(); |
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try { |
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return parties - count; |
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} finally { |
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lock.unlock(); |
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} |
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} |
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} |